Process of forming a deflection mirror in a light waveguide

ABSTRACT

A process of forming a deflection mirror in a light waveguide with a use of a dicing blade having a cutting end with a flat top cutting face and at least one slanted side cutting face. The process includes a cutting step of cutting a surface of the light waveguide to a depth not greater than a width of the flat top cutting face, thereby forming a groove in the surface of the light waveguide. The groove has a slanted surface which is formed by the slanted cutting face to define the deflection mirror in the waveguide.

TECHNICAL FIELD

The present invention is directed to a process of forming a deflectionmirror in a light waveguide of planar structure.

BACKGROUND ART

In order to change a direction of light proceeding into or out of aplanar light waveguide, the light waveguide is formed with a groove witha slanted surface which defines a deflection mirror. The groove isformed by a process for example disclosed in Japanese Patent PublicationNo. 10-300961, in which a dicing blade is utilized to cut a surface ofthe light waveguide to give the groove. The dicing blade utilized in thepublication is configured to have a V-shaped cutting end with a noseangle of about 90° or a wedge-shaped cutting end with a nose angle ofabout 45°. In view of that the light waveguide is processed to give thegroove normally having a depth of 100 μm, the dicing blade having thesharp cutting edge is susceptible to being damaged, and also to afluctuation while being driven to rotate and advance along a straightpath, which results in excessive wearing of the blade and thereforemaking the resulting groove inaccurate. Further, since the dicing bladeis placed perpendicular to the surface of the light waveguide, theresulting deflection mirror will be easy to deviate from an intendedpotion as the dicing blade is caused to vary a cutting depth.

SUMMARY OF THE INVENTION

The present invention has been achieved to provide an improved processof forming a deflection mirror in a light waveguide which is capable ofeliminating the above problem. The process in accordance with thepresent invention utilizes a dicing blade having a cutting end with aflat top cutting face and at least one slanted side cutting face, andcomprises a cutting step of cutting a surface of the light waveguide toa depth not greater than a width of the flat top cutting face so as toform therein a groove having a slanted surface which is formed by theslanted side cutting face to define the deflection mirror. With theprovision of the flat top cutting face, the dicing blade utilized in theprocess can be therefore given a sufficient strength against the wearingof the side cutting face as well as undesired fluctuation while beingrotated, thereby keeping accuracy of the resulting groove over aprolonged use of repeating the cutting processes.

Preferably, the dicing blade includes in its surface abrasive granuleshaving an abrasion scale of 4500 to 6000 according to the JapaneseIndustry Standard R6001 for smoothening the deflection mirror.

Further, in order to precisely control the depth of the groove, i.e., aposition of the deflection mirror in the light waveguide, the cuttingstep may include sub-steps of lowering the dicing blade relative to thelight waveguide to a predetermined first level to form a preliminarygroove having a depth less than a final depth intended to be given tothe groove, and releasing the dicing blade away from the lightwaveguide. Then, it is made to measure an open-end width of thepreliminary groove to obtain an actual depth of the preliminary groovewhen the dicing blade is lowered to the first level. The actual depth iscalculated from the open-end width and a known geometrical configurationof the cutting end of the dicing blade. Then, it is made to calculate atarget level where the dicing blade operates to cut the light waveguideto an intended final depth so as to form the groove. The target level isobtained in terms of the first level, the actual depth of thepreliminary groove and the indented final depth. Subsequently, thedicing blade is lowered to thus obtained target level to form the groovewith the deflection mirror precisely at the intended position.

When forming a plurality of deflection mirrors respectively in aplurality of the light waveguides horizontally arranged in a parallelrelation with each other, it is preferred to control the target levelfor each of the deflection mirrors. In this instance, the dicing bladeis lowered to the predetermined level at a plurality of pointsrespectively meeting with the light waveguides to form the preliminarygroove at each of the points. After releasing the dicing blade away fromthe light waveguides, it is made to measure an open-end width at eachone of points where deflection mirrors are formed, and to calculate theactual depth at each point of the preliminary groove. Then, the targetlevel of the dicing blade at each point is obtained based upon theactual depth, the first level, and the final depth. Subsequently, thedicing blade is controlled to advance along a straight pathperpendicular to the length of the waveguides while continuously varyinga cutting depth towards the target levels at the points, so as to cutthe groove having the final depth. The preliminary grooves may be madecontinuous along the straight path by advancing the dicing blade alongthe straight path.

In a preferred embodiment, the light waveguide is composed of a firstclad layer, a core layer superimposed on the first clad layer, and asecond clad layer superimposed on the first clad layer over the corelayer and merging into the first clad layer to surround the core layer.In this instance, the cutting process is applied to cut the core layerprior to being covered by the second clad layer.

Further, the process of the present invention may include an additionalstep of irradiating an energy beam to the slanted surface of the groovefor smoothening the resulting deflection mirror, or of coating theslanted surface of the groove with a resin for smoothening the resultingdeflection mirror. When irradiating the energy beam, a flat bottomformed in the groove by the flat top cutting face gives sufficient spacewhich enables to irradiate the energy beam along a direction normal tothe slanted surface for effective smoothening of the resultingdeflection mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a set of sectional views illustrating a process in accordancewith a first embodiment of the present invention;

FIG. 2 illustrates a section of a cutting end of a dicing blade utilizedin the above process;

FIG. 3 is a set of sectional views illustrating a process in accordancewith a second embodiment of the present invention;

FIG. 4 illustrates a section of a cutting end of a dicing blade utilizedin the above process of FIG. 3;

FIG. 5 is a sectional view of a portion of a light waveguide to betreated by the above process;

FIG. 6 is a graph illustrating a relation between a surface roughnessand a running speed of the dicing blade;

FIG. 7 is a graph illustrating a relation between a surface roughnessand a grain size of abrasive granules included in the dicing blade;

FIG. 8 is a plan view illustrating a straight path along which thedicing blade advances to form a plurality of deflection mirrors alongthe straight path in accordance with a third embodiment of the presentinvention; and

FIG. 9 is a set of sectional views illustrating a process for forming aplurality of the deflection mirrors of the above embodiment of FIG. 8.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1 and 2, there is shown a process of forming adeflection mirror 14 in a light waveguide 10 in accordance with a firstembodiment of the present invention. The light waveguide 10 is formed ona substrate 20 and is composed of a core layer 13 surrounded by firstand second clad layers 11 and 12. The first and second clad layers areboth made of an ultraviolet curable transparent resin having arefractive index of 1.52, while the core layer 13 is made of a likeultraviolet curable transparent resin having a refractive index of 1.54.The first clad layer 11 is formed by spin-coating the UV-curable resinon the substrate 20 and curing the resin by exposure to an ultravioletirradiation to have a thickness of about 20 μm. The core layer 13 isformed by coating the UV-curable resin on the first clad layer 11 andcuring the resin by selective exposure to the ultraviolet irradiationthrough a mask to have a predetermined pattern having a thickness ofabout 40 μm. Uncured portion of the resin is washed out by an organicsolvent (FIG. 1(B)). Thereafter, the UV-curable resin is applied overthe first clad layer 11 over the core layer 13 and is cured by exposureto the ultraviolet irradiation to form the second clad layer 12 whichmerges with the first clad layer 11 to surround the core layer 13, asshown in FIG. 1(C), thereby establishing the light waveguide 10 of aplanar structure. The UV resin may include an epoxy-based resin or anyother resin such as PGMEA (Propylene Glycol Monomethyl Ether Acetate).

The deflection mirror 14 is formed in the light waveguide 10 to reflecta light passing through the core layer 13 to a direction perpendicularto a plane of the light waveguide 10 or to reflect an incoming light tothe core layer 13. In the illustrated instance, two deflection mirrors14 are realized by two grooves 30 respectively formed in the oppositeends of the clad layers 13. Each of the grooves 30 is formed by cuttingthe top surface of the light waveguide 10 with the use of a dicing blade40 to have a flat bottom 31, an upright side 32, and an inclined sideopposed to the upright side to define the deflection mirror 14. As shownin FIG. 2, the dicing blade 40 has a cutting end of which cross-sectionis analogous to that of the groove 30, and includes a flat top cuttingface 41, a straight side cutting face 42, and a slanted side cuttingface 44. The fat top cutting face 41 is dimensioned to have a width (W)of 50 μm or greater, when the groove 30 is formed to have a depth (D) of50 μm. The slanted side cutting face 44 is inclined at an angle of 45°with respect to the flat top cutting face 41, and has a height (H)greater than the width (W). The dicing blade 40 is lowered while beingdriven to rotate with its straight side cutting face 42, i.e., a planeof the dicing blade normal to the light waveguide 10 in order to cut thegroove 30 of which depth (D) is not greater than the width (W) of theflat top cutting face 41.

Having the flat top cutting face 41, the dicing blade 40 is given anincreased strength at its cutting end bearing a maximum cuttingresistance, such that the cutting end can be kept in an intendedposition to precisely develop the groove 30 and therefore the deflectionmirror 14. Especially, by limiting the depth (D) of the groove 30 notgreater than the width (W) of the flat top cutting face 41, the dicingblade 40 can be kept from receiving an excessive cutting resistance atits cutting end, and being therefore free from fluctuation or damageduring the cutting operation to give the precise groove, while assuringa prolonged use of the blade.

FIGS. 3 and 4 illustrate a second embodiment of the present invention inwhich the cutting is made prior to the core layer 13 being covered bythe second clad layer 12 and with the use of a dicing blade 40 havingdouble-sided slanted cutting faces. The dicing blade 40 has a cuttingend defined by a flat top cutting face 41, and an opposed pair ofslanted side cutting faces 44. The flat top cutting face 41 has a width(W) which is greater than a depth (D) of the groove cut into the cladlayer 13. Each of the slanted side cutting faces 44 is inclined at anangle of 45° with respect to the flat top cutting face 41 and has aheight (H) greater than the width (W) of the top cutting face 41. Inthis instance, after the core layer 13 is formed on the laminate of thefirst clad layers 11, and before the second clad layer 14 is formed tocover the core layer 13, the dicing blade 40 is lowered to cut the corelayer 13 directly to give a corresponding groove 30 with a pair ofdeflection mirrors 14 in the core layer 13. The groove 30 is given adepth (D) equal to the thickness of the core layer 30 which is less thanthe width (W) of the flat top cutting face 41. After releasing thedicing blade 40 away from the core layer 13, the UV resin is applied onthe upper first clad layer 11 over the core layer 13 to form a secondclad layer 12 which merges with the upper first clad layer 11 tosurround the core layer 13.

With the direct cutting of the core layer 13, the cutting can be freefrom a possible variation in a thickness of the clad layer 12 coveringthe core layer 13, as shown in FIG. 5, and can be made accurately to anintended depth. Further, since the dicing blade 40 experiences lesscutting resistance than in a case of cutting the core layer through theclad layer, an intended smooth finish can be given to the resultingdeflection mirror 14 at a reduced speed of driving the dicing blade 40.For example, an intended smoothness of the mirror, i.e., a surfaceroughness of less than 110 rms (nm) is obtained only at a driving speedof around 1.0 mm/s while cutting the core layer through the clad layer,the same smoothness can be obtained also at an increased speed of around10.0 mm/s, as shown in FIG. 6, while cutting the core layer directly,i.e., without cutting the clad layer. This reduces a processing time ofcutting the groove about 10 times faster. The driving speed is usedherein to mean a speed of advancing the dicing blade rotating at aconstant rotating speed, for example, 15,000 rpm, along one straightpath across the light waveguide and also a speed of lowering the dicingblade into the surface of the light waveguide.

In anyone of the above embodiments, the dicing blade 40 has its cuttingface finished with diamond granules selected to have an abrasion scaleof #4500 to #5000 (equivalent to an average grain size of 2.0 μm to 4.0μm) in accordance with the JIS (Japanese Industrial Standard) R6001, inorder to give the intended surface smoothness to the deflection mirror14, as shown in FIG. 7. If the granules having the average grain sizeless than the above range are used, the abrasion would be insufficientto give only a poorly polished face to the resulting mirror.

The deflection mirror may be also polished or smoothed by exposure to anenergy beam irradiation. The irradiation of the energy beam is effectiveto ablate minute surface irregularity possibly remaining on the mirrorsurface. An infrared laser is found advantageous as it provides its highintensity beam with an easy handling. In view of that the UV-curableresin utilized for the light waveguide has a molecular vibrationabsorption at a wavelength of around 10 μm, a carbon dioxide laser (CO₂laser) generating the laser beam of such wavelength is particularlyadvantageous.

In order to further improve reflectivity, the mirror may be finishedwith a reflective film made of a metal or dielectric multi-layeredcoating to be formed by plating or spattering deposition. Suchreflective film may be also effective to reflect the light to a specificdirection which is not possible by a total reflection.

For instance, the slanted surface or the deflection mirror 14 formed atthe step of FIG. 1(E) is treated with a TEA-CO₂ laser beam having thewavelength of 9.8 μm which is irradiated along a direction normal to themirror 14 with an energy density of 9 mJ/mm², four (4) irradiationpulses having a pulse width of 9.3 μs, and a repeated frequency of 100Hz to an irradiation area of 100 μm². After the laser irradiation, thesurface roughness is improved from 100 nm (rms) to 50 nm (rms). Then,the reflective film of gold is deposited on the deflection mirror. Forevaluation of reflection loss, the waveguide is cut at a point spacedinwardly from the upright side 32 by a distance of 1 cm so as to measureintensity of light which is incident on the mirror 14 and that comingoutwardly of that point after being reflected at the mirror. Theresulting reflection loss is found to be 0.7 dB at a wavelength of 670μm.

It is noted in this connection that the irradiation of the laser beam ina direction normal to the mirror 14 is made easy and possible with thepresence of the flat bottom 31 in the groove 30 of which width (W) isnot less than the depth (D) of the groove.

Alternatively, the mirror 14 may be finished with a resin coating whichhas the same refractive index as the core layer 13 and is made of a likeresin material as the core layer or the clad layer. The resin coating ismade by applying a diluted solution of the resin and subsequently curingit. Also, in this instance, the flat bottom 31 of the groove 30 can holdan excess amount of the solution to leave the cured coating of uniformthickness only on the deflection mirror 14 opposite to the core layer13. For instance, when the core layer 13 is made of PGMEA (PropyleneGlycol Monomethyl Ether Acetate), a varnish solution containing 2 wt %of PGMEA is utilized to coat the mirror followed by being selectivelycured by exposure to the UV light beam to give the resin coating ofuniform thickness. Thereafter, the reflective film of gold is depositedon the resin coating. Also, the like evaluation of the reflection lossis made in a manner as described in the above to give a result that thereflection loss is found to be 0.8 dB at a wavelength of 670 μm. Theresin coating is advantageous in its compatibility to the core layer andthe clad layer in terms of thermal expansion coefficient.

The method of the present invention is used to cut the grooves in aplurality of the core layers 13 or waveguides commonly formed on thesubstrate 20 while advancing the dicing blades along a straight path Pperpendicular to the length of the core layers 13, as shown FIG. 8. Thecore layers 13 are arranged horizontally in a parallel relation witheach other. In consideration of a possible variation of a height of thetop surfaces among the plurality of the light guides 10, a control ismade to lower the dicing blade to an exact level of forming the grooveof the intended depth D at each of points Y₀ to Y_(n) crossing with thecore layers 13 when advancing the dicing blade along the straight pathP. For this purpose, it is first made to lower the dicing blade withrespect to the light waveguide 10 to a predetermined first level Z₀ andadvance it along the straight path P while keeping it at the first levelZ₀ in order to form a preliminary groove 30P, as shown in FIG. 9(A). Thefirst level Z₀ is measured from a reference level and is selected togive a depth D₀ which is less than the final depth D of the groove 30,for instance about half of the final depth D. After removing the dicingblade 40 away from the waveguide, it is made to measure an open-endwidth W₀ of the preliminary groove 30P, as show in FIG. 9(B), for eachof the points Y₀ to Y_(n), so as to calculate a corresponding depth D₀of the preliminary groove 30P for each point based upon the open-endwidth W₀ and a known geometrical configuration of the cutting end of thedicing blade 40, and to calculate a target level Z₁ which is the firstlevel Z₀ minus a difference between the intended final depth D and thedepth D₀ of the preliminary groove, i.e., Z₁=Z₀−(D−D₀). The target levelZ₁ for each of the points Y₀ to Y_(n) is stored in a memory. Next, thedicing blade 40 is controlled to advance in the preliminary groove 30Palong the straight path P as being lowered respectively to the targetlevel Z₁, as shown in FIG. 9(C), determined respectively to thepositions Y₀ to Y_(n) before the dicing blade 40 reaches the individualpoints. In this manner, each of the waveguides 10 can be cut to aprecisely controlled depth D to make the resulting deflection mirror 14in an exact relation to the end of the core layer 13. When the finaldepth D of the groove 30 is 50 μm, the first level Z₀ is selected togive the preliminary groove 30P having the depth D₀ of about 30 μm.Although the preliminary groove 30P is made continuous along thestraight path P in the above embodiment, it is equally possible to cutthe light waveguides only at the points Y₀ to Y_(n) for the purpose ofcalculating the target level for each of the points Y₀ to Y_(n).

Although the above control is particularly advantageous for cutting theplurality of the waveguides arranged along the straight path, it isequally applicable for cutting the single waveguide. Further, althoughthe above illustrated embodiments are shown to cut the groove 30 in thewaveguide 10 of planar configuration in which the second clad layer 12is superimposed on the first clad layer 12 to merge into the first cladlayer to surround the core layer 13 superimposed on the first cladlayer, the present invention should be not limited to this specificembodiment and be equally applicable to the light waveguide comprisingone or more optical fibers mounted on a substrate.

1. A process of forming deflection mirrors in a plurality of light waveguides, said process comprising the steps of: advancing a dicing blade across said plurality of light waveguides while lowering said dicing blade to a predetermined first level of an initial depth at least at a plurality of points where said light waveguides meet so as to make a preliminary cut in each of said light waveguides to form a preliminary groove having an initial depth, releasing said dicing blade away from said light waveguides, measuring an open-end width of the preliminary groove at each of said points to (1) calculate the initial depth of said preliminary groove based upon said open-end width and a geometrical configuration of the dicing blade, and (2) calculate a target level for each of said points in terms of said predetermined first level, said initial depth of the preliminary groove, and a final depth of a groove, said target level being defined as a distance from said first level minus a difference between the initial depth of said preliminary groove and said final depth of said groove; and advancing said dicing blade along a straight path across said plurality of light waveguides while lowering said dicing blade to said target level at each of said points to cut said groove having said final depth and a slanted surface to define said deflection mirrors, wherein the initial depth of the preliminary groove is less than the final depth of the groove, wherein said plurality of light waveguides are arranged in a horizontal plane and spaced in a parallel relation with each other, wherein said dicing blade has a cutting end with a flat top cutting face and at least one slanted side cutting face, wherein said final depth is not greater than a width of said flat top cutting face, and wherein said slanted surface of said groove is formed by said slanted side cutting face of said dicing blade.
 2. A process as set forth in claim 1, wherein said preliminary cut is made along a straight path to make said preliminary grooves continuous along said straight path.
 3. A process as set forth in claim 1, wherein said lowering of said dicing blade to said target level at each of said points is adjusted to account for variations of height of top surfaces of each said light waveguides. 